From the Gaelic:Mac Gille breith -or- Mac Gille bhrath"Son of the Servant of Judgement"

The above Gaelic orthography for the name MacGillivray is only one of a number that have been suggested, though it is the one most widely accepted. In the context of the Gaelic social structure of the west coast and Isles of Scotland from whence the Clan is thought to have originated, it most likely refers to an occupational role, and perhaps a hereditary one, of 'deemster' (or doomsayer), the officer of the lord's court of law who pronounces its final judgement.

This would be consistent with the way that civil administration and professions were arranged under the MacDonald Lords of the Isles (c.1158-1493) over the whole western and insular section of Scotland where their writ held sway. This dominion of 'Gaeldom' was distinct in many of its forms and often autonomous in its power from the progressively Anglo-Norman Kings of Scots whose rule held elsewhere in Scotland.

That MacGillivrays were an old clan within this society is affirmed in Clan Donald's own traditions, and that they had some prominence in it is attested by the Seannachaidh Hugh MacDonald of Sleat, who counts 'MacGillevray in Mull' among the freeholders entitled to sit on the Council of the Isles, assembled at Loch Finlaggan in Islay and elsewhere.